The present invention relates to a nitrogen oxide storage material which contains at least one storage component for nitrogen oxides in the form of an oxide, carbonate or hydroxide of an alkaline earth metal selected fro the group consisting of magnesium, calcium, strontium and barium and an alkali metal selected from the group consisting of potassium and cesium on a support material.
In the gasoline engine sector, so-called lean engines have been developed to reduce the consumption of fuel. These engines operate with lean air/fuel mixtures when not operating under full load. A lean air/fuel mixture contains a higher oxygen concentration than is required for complete combustion of the fuel. In the corresponding exhaust gas the oxidizing components oxygen (O2) and nitrogen oxides (NOx) are then present in an excess as compared with the reducing exhaust gas components carbon monoxide (CO), hydrogen (H2) and hydrocarbons (HC). Lean exhaust gas usually contains 3 to 15 vol. % of oxygen. However, when operating under full or relatively full load, even lean operating gasoline engines have a stoichiometric ratio or even less than stoichiometric ratio, that is to say a rich, air/fuel composition.
Diesel engines on the other hand operate under all conditions with well above stoichiometric air to fuel mixtures.
Due to the high oxygen content of exhaust gas from lean engines or diesel engines, the nitrogen oxides contained therein cannot be reduced, as is the case with stoichiometrically operated gasoline engines, by using so-called three way catalytic converters with simultaneous oxidation of hydrocarbons and carbon monoxide to yield nitrogen.
To remove nitrogen oxides from these exhaust gases, therefore, nitrogen oxide storing catalysts have been produced which store the nitrogen oxides contained in lean exhaust gas in the form of nitrates.
The mode of operation of nitrogen oxide storing catalysts is described in detail in SAE 950809 which is incorporated herein by reference. Accordingly, nitrogen oxide storing catalysts consist of a catalyst material which is generally applied in the form of a coating on an inert honeycomb structure made of ceramic or metal, a so-called carrier structure. The catalyst material contains the nitrogen oxide storage material and a catalytically active component. The nitrogen oxide storage material thus consists of the actual nitrogen oxide storage component which is deposited onto a support material in highly dispersed form.
The basic oxides of alkali metals, alkaline earth metals and rare earth metals, preferably barium oxide, are used as storage components which react with nitrogen dioxide to give the corresponding nitrates. It is known that these oxide materials are largely present, in the presence of air, in the form of carbonates and hydroxides. These compounds are also suitable for storing nitrogen oxides. When therefore in the context of the invention the term xe2x80x9cbasic storage oxidesxe2x80x9d is referred to, then this expression is intended to also include the corresponding carbonates and hydroxides.
The noble metals from the platinum group of metals from the Periodic Table of Elements are generally used as catalytically active components and these are generally deposited on to the support material together with the storage component. Active, high surface area aluminum oxide is generally used as support material as is well known in this art.
The task of the catalytically active component is to convert carbon monoxide and hydrocarbons in lean exhaust gas to carbon dioxide and water. In addition they should oxidize any nitrogen monoxide in the exhaust gas to nitrogen dioxide so that it can react with the basic storage material to give nitrates. With increasing incorporation of nitrogen oxides in the storage material the storage capacity of the material decreases and it therefore has to be regenerated from time to time. For this purpose, the engine is operated for short periods with a stoichiometric composition or a rich air/fuel mixture. Under the reducing conditions in a rich exhaust gas the nitrates which have been produced decompose to give nitrogen oxides NOx and are reduced to nitrogen, with the production of water and carbon dioxide, by using carbon monoxide, hydrogen and hydrocarbons as reducing agents. The storing catalyst operates as a three way catalytic converter during this operating phase.
A substantial problem with storage materials is their inadequate resistance to ageing at high temperatures. As pointed out in SAE Technical Paper 970746, an important ageing mechanism for nitrogen oxide storage materials comprises the actual storage component reacting with the support material. Thus, when a storage material consisting of barium oxide on zirconium oxide was aged for a period of 24 hours at 750xc2x0 C. the production of barium zirconate BaZrO3 was observed. Barium oxide on titanium oxide led to the production of barium titanate. In both cases this reaction of storage component with support material was associated with a high loss of nitrogen oxide storage capacity. Zirconium oxide and titanium oxide are thus unsuitable as supports for alkali metal and alkaline earth metal storage components due to their high tendency to react with barium oxide when they are subjected to high temperatures during use. Aluminum oxide behaved somewhat better as a support material, but even here the production of barium aluminate occurred with long term ageing at high temperatures.
Various combinations of storage components and support materials which are also intended to solve this ageing problem have been disclosed in the patent literature. Thus, EP 0 562 516 A1 describes a catalyst consisting of barium oxide, lanthanum oxide and platinum on a support material made of aluminum oxide, zeolite, zirconium oxide, aluminum silicate or silicon dioxide, wherein at least some of the barium oxide and lanthanum oxide form a mixed oxide. By virtue of this mixed oxide the production of lanthanum aluminate, which would otherwise lead to ageing of the catalyst, is intended to be suppressed.
In order to suppress the reaction of storage components with a support consisting of aluminum oxide EP 0 645 173 A2 proposes dissolving lithium in the support in such a way that a solid solution of aluminum oxide and lithium is formed.
EP 0 653 238 A1 suggests as support material, titanium oxide which contains at least one element selected from the group of alkali metals, alkaline earth metals and rare earth metals in the form of a solid solution.
EP 0 657 204 A1 discloses the mixed oxides TiO2xe2x80x94Al2O3, ZrO2xe2x80x94Al2O3 and SiO2xe2x80x94Al2O3 as support materials for nitrogen oxide storing catalysts. In addition, mixed oxides of TiO2, Al2O3 with alkaline earth metals and rare earth metals, in particular TiO2xe2x80x94Al2O3xe2x80x94Sc2O3, TiO2xe2x80x94Al2O3xe2x80x94Y2O3, TiO2xe2x80x94Al2O3xe2x80x94La2O3 and TiO2xe2x80x94Al2O3xe2x80x94Nd2O3 are mentioned as support materials.
EP 0 666 103 A1 describes a catalyst which contains a nitrogen oxide storage component and a noble metal on a porous support material. Aluminum oxide, zeolite, zirconium oxide, aluminum silicate and silicon dioxide are suggested as support material. The nitrogen oxide storage component and noble metal are deposited onto these support particles in very close association. In addition, the catalyst may also contain cerium oxide as an oxygen storage component, wherein cerium oxide is kept separate from the noble metal and thus also from the storage component.
EP 0 718 028 A1 discloses a thermally resistant nitrogen oxide storage material. The high thermal resistance is obtained by finely dispersing the nitrogen oxide storage component in the support material. For this purpose, a solution of a compound of at least one alkali metal, one alkaline earth metal and one rare earth metal is mixed with a solution of an oxide sol of at least one metal selected from Groups IIIb, IVa and IVb of the Periodic Table of Elements, and converted into a gel, dried and calcined. The resulting storage material is amorphous. In the examples this storage material is combined inter alia with a catalyst powder which contains platinum on a high surface area cerium/zirconium mixed oxide. The cerium/zirconium mixed oxide thus forms the support material for the platinum component in this case.
EP 0 771 584 A1 also describes a thermally resistant support material for catalysts which also consists of an amorphous mixed oxide. The amorphous mixed oxide is composed of a nitrogen oxide storage component selected from the group of alkali metals, alkaline earth metals and rare earth metals and also of aluminum oxide and at least one oxide selected from the group of titanium oxide, zirconium oxide and silicon dioxide. The aluminum oxide is an important constituent of the amorphous mixed oxide and is present in a molar ratio of 4 to 12 compared with the storage component. The support material may also contain cerium oxide as an oxygen storing material. Cerium oxide and nitrogen oxide storage components may only be present in a molar ratio to each other between 0.5 and 3 in the support material. Outside these limits the thermal resistance is impaired according to data from EP 0 771 584 A1.
WO 97/02886 describes a nitrogen oxide storing catalyst in which the storage component and the catalytically active component are spatially separated from each other but located adjacent to each other. For this purpose, storage component and catalytic component are applied in two superimposed layers on a support structure. Alternatively, storage component and catalytic component may be deposited on different support particles which are then applied together in the form of a coating on the carrier structure. Metal oxides, metal hydroxides, metal carbonates and metal mixed oxides are described as storage materials. The metals may be lithium, sodium, potassium, rubidium, caesium, magnesium, calcium, strontium or barium.
The storage material in accordance with WO 97/02886 may contain a sulfur absorbing component, preferably cerium oxide, to prevent poisoning by sulfur. This cerium oxide may be present in the form of particles alongside the particles of the storage material or be dispersed in the nitrogen oxide storage component.
None of the proposed solutions described above provides nitrogen oxide storage materials with adequate resistance to ageing. Rather, in the case of all known storage materials, deactivation of the alkali metal and alkaline earth metal oxides which store nitrogen oxides takes place due to irreversible reaction with the support materials to give the corresponding aluminates, zirconates, silicates, titanates etc. at elevated temperatures ( greater than =700xc2x0 C.). And then, the alkali metal and alkaline earth metal oxides are no longer capable of storing nitrogen oxides due to the loss of basicity which results. Since the storage components are generally present in a molar excess as compared with the support oxide, the storage components can react completely to produce a mixed oxide with the support oxide, depending on the time, the exhaust gas composition and the exhaust gas temperature.
German patent publication DE 197 13 432A1 describes a catalytic base material for an exhaust gas purification catalyst which is obtained by impregnating cerium oxide with a solution containing barium and calcining the cerium oxide particles at a temperature from 400 to 1100xc2x0 C. to form barium oxide at the surface of the cerium oxide particles. According to this document the mixture of barium oxide and cerium oxide particles is calcined at a relatively high surface of the cerium oxide particles. For that purpose temperatures of from 800 to 1100xc2x0 C. are very effective. Preferably the cerium oxide particles are calcined at 900xc2x0 C. for a period of 24 hours. This produces particles of barium oxide with a particle size between 7 and 14 xcexcm. At a calcination temperature of 200xc2x0 C. the mean particle size is still 1.9 xcexcm.
The catalytic base material according to DE 197 13 432A1 is used for the manufacture of a catalyst which is especially effective for purification of the exhaust gas of lean burn combustion engines. Therefore, this catalyst is a so-called lean-NOx catalyst which is able to reduce nitrogen oxides to nitrogen in a lean exhaust gas provided the exhaust gas contains a sufficient amount of reductive exhaust gas components (carbon monoxide and hydrocarbons). The catalytic base material is said to enhance the temperature stability of the catalyst. Nothing is said in DE 197 13 432A1 about the nitrogen oxides storage capability of the catalytic base material.
Accordingly, it is an object of the present invention to provide a nitrogen oxide storage material which is characterized by a high degree of storage efficiency and also substantially improved resistance to ageing.
Another object of the invention is to provide for preparing the storage material and a process to prepare a nitrogen oxide store on an inert support prepared from the storage material.
A further object is to prepare a nitrogen oxide storing catalyst by combining the storage material with noble metals from the platinum group of the Periodic Table of Elements.
A still further object of the present invention is to treat exhaust gas from lean operating internal combustion engines using the nitrogen oxide store and the storing catalyst.
The above and other objects can be achieved by a nitrogen oxide storage material which contains at least one storage component for nitrogen oxides in the form of particles of an oxide, carbonate or hydroxide of an alkaline earth metal selected from the group consisting of magnesium, calcium, strontium and barium and an alkali metal selected from the group consisting of potassium and cesium on a support material.
The nitrogen oxide storage material is characterized in that the support material is selected from the group consisting of doped cerium oxide, cerium/zirconium mixed oxide, calcium titanate, strontium titanate, barium titanate, barium stannate, barium zirconate, magnesium oxide, lanthanum oxide, praseodymium oxide, samarium oxide, neodymium oxide, yttrium oxide, yttrium barium cuprate, lead titanate, tin titanate, bismuth titanate, lanthanum cobaltate, lanthanum manganate, barium cuprate and mixtures thereof. In addition, the particles of the nitrogen oxide storage components have a mean particle diameter of less than 1.5 xcexcm. The support materials to be used can be divided into four groups. The first group includes support materials based on cerium oxide. These may be doped cerium oxide or cerium/zirconium mixed oxides. Independently of the type of preparation, these materials have a largely crystalline structure, and are thus not amorphous.
The second group includes stoichiometrically composed mixed oxides comprising the oxides of the storage components and support oxides. Barium titanate may be mentioned as an example as a mixed oxide of barium oxide and titanium oxide. This group thus includes calcium titanate, strontium titanate, barium titanate, barium stannate and barium zirconate.
The third group comprises the pure oxides magnesium oxide, lanthanum oxide, praseodymium oxide, samarium oxide, neodymium oxide and yttrium oxide.
The fourth group comprises the other mixed oxides, yttrium barium cuprate, lead titanate, tin titanate, bismuth titanate, lanthanum cobaltate, lanthanum manganate and barium cuprate.
It has been found that an active and ageing-resistant storage material is obtained when the support material used for the storage component has only a low or zero tendency to react with the storage component and is also stable under the alternating lean and rich exhaust gas compositions of lean engines; that is, the specific surface area of the support materials should be largely stable under the actual exhaust gas conditions of lean engines.